Semiconductor devices for detecting physical quantity distribution have found wide application in a variety of fields. Such devices have a plurality of unit components (e.g., pixels), sensitive to externally fed electromagnetic waves such as light and radiation, arranged in lines or in a matrix form.
In the field of video equipment, for example, CCD (Charge Coupled Device) or MOS (Metal Oxide Semiconductor) and CMOS (Complementary Metal Oxide Semiconductor) solid-state imaging devices are used. These devices are designed to detect light (an example of electromagnetic waves) among other physical quantities. Such devices allow a physical quantity distribution, converted into an electric signal by unit components (pixels in a solid-state imaging device), to be read out therefrom in the form of an electric signal.
Some solid-state imaging devices are amplifying devices. These devices include, in a pixel signal generation section, pixels configured as solid-state imaging elements (APSs: Active Pixel Sensors, also referred to as gain cells), each of which has an amplifying drive transistor. The pixel signal generation section generates a pixel signal commensurate with the charge of the signal generated by a charge generation section. For example, many of CMOS solid-state imaging devices are configured in such a manner.
In order to read out a pixel signal externally from such an amplifying solid-state imaging device, a pixel section having an arrangement of a plurality of unit pixels is address-controlled so that the signal from a unit pixel can be arbitrarily selected and read out. That is, an amplifying solid-state imaging device is an example of address-controlled solid-state imaging device.
A unit pixel includes a charge generation section and signal output section. The charge generation section generates a signal charge. The signal output section has a transistor adapted to generate and output a target signal which is commensurate with the signal charge generated by the charge generation section. For example, the charge generation section has a photodiode which performs photoelectric conversion. The signal output section has a readout selection transistor, amplifying transistor, reset transistor and selection transistor. The readout selection transistor reads out the signal charge generated by the photodiode. The amplifying transistor converts the read signal charge into a pixel signal. The reset transistor resets the signal charge. The selection transistor selects the pixel to be read out.
Here, MOS solid-state imaging devices have a dark current problem caused by a leak phenomenon in which the signal charge generated by the charge generation unit leaks out into the signal output side. If the signal charge is accumulated for long hours in particular, the dark current component increases cumulatively, thus accounting for a large proportion of the signal charge. The dark current component cannot be separated from the signal charge during readout. The variation therein results in noise, significantly degrading the image quality. For example, the variation in the dark current component from one pixel to another leads to fixed pattern noise, causing white dots to appear in the image. As a result, the image looks as if it was captured through ground glass. Further, the variation in the dark current component over time results in random noise. In the case of MOS solid-state imaging devices, therefore, it is a matter of concern how to reduce the dark current component.
As a countermeasure thereagainst, for example, Republished Patent Application No. WO2003/085964 proposes an arrangement for reducing dark current. The arrangement brings the voltage applied to the gate (referred to as the transfer gate) of the readout selection transistor to the ground potential or less, that is, applies a negative voltage to the transfer gate, thus accumulating holes in the transfer gate channel and providing reduced dark current.
However, if a negative voltage is used as described above, excessively lowering the negative voltage of the transfer gate, that is, negatively increasing the voltage of the transfer gate, results in more stress on the pixel and the gate oxide film of its drive circuit. Further, the transistor characteristics degrade, for example, due to hot carriers, significantly affecting the reliability (product life). On the other hand, increasing the negative voltage (negatively reducing the voltage) in consideration of reliability results in failure to prevent leaks, exacerbating noise caused by dark current in the case of long hours of accumulation. Thus, at present, both reliability and leak phenomenon cannot be met sufficiently. Therefore, it is difficult to optimize the negative voltage at a constant level.
Possible solutions to use a constant negative voltage level would be to increase the thickness of the gate oxide film of the transistors to which a negative voltage is applied and to enhance the reliability (durability). However, this leads to an increased number of manufacturing steps, thus resulting in increased cost.
The present invention has been made in light of the foregoing problems, and it is an object of the present invention to reduce dark current caused by leak while at the same time ensuring element reliability using a simple arrangement.